Cohort

Cohort is a Spring Actuator style replacement for Ktor. It provides HTTP endpoints to help monitor and manage apps in production. For example healthchecks, logging, database and JVM metrics.

See changelog

Features

All features are disabled by default.

• Comprehensive system healthchecks: Expose healthcheck endpoints that check for thread deadlocks, memory usage, disk space, cpu usage, garbage collection and more.
• Resource healthchecks: Additional modules to monitor the health of Redis, Kafka, Elasticsearch, databases and other resources.
• Database pools: See runtime metrics such as active and idle connections in database pools such as Hikari Connection Pool.
• JVM Info: Enable endpoints to export system properties, JVM arguments and version information, and O/S name / version.
• Thread and heap dumps: Optional endpoints to export a thread dump or heap dump, in the standard JVM format, for analysis locally.
• Database migrations: See the status of applied and pending database migrations from either Flyway or Liquibase.
• Logging configuration: View configured loggers and levels and modify log levels at runtime.

How to use

Include the following dependencies in your build:

• com.sksamuel.cohort:cohort-core:<version>

And either

• com.sksamuel.cohort:cohort-ktor:<version> for ktor 1.x or
• com.sksamuel.cohort:cohort-ktor2:<version> for ktor 2.x

along with the additional modules for any features you wish to activate. For example the kafka module requires com.sksamuel.cohort:cohort-kafka:<version>.

Then to wire into Ktor, install the Cohort plugin, and enable whichever features / endpoints we want to expose.

Here is a sample configuration with each feature enabled.

install(Cohort) {

  // enable an endpoint to display operating system name and version
  operatingSystem = true

  // enable runtime JVM information such as vm options and vendor name
  jvmInfo = true

  // configure the Logback log manager to show effective log levels and allow runtime adjustment
  logManager = LogbackManager

  // show connection pool information
  dataSources = listOf(HikariDataSourceManager(ds))

  // show current system properties
  sysprops = true

  // enable an endpoint to dump the heap in hprof format
  heapdump = true

  // enable an endpoint to dump threads
  threaddump = true

  // enable healthchecks for kubernetes
  // each of these is optional and can map to any healthchecks/url you wish
  healthcheck("/liveness", livechecks)
  healthcheck("/readiness", readychecks)
  healthcheck("/startup", startupchecks)
}

Healthchecks

Cohort provides HealthChecks for a variety of JVM metrics such as memory and thread deadlocks as well as connectivity to services such as Kafka and Elasticsearch and databases.

We use health checks by adding them to a HealthCheckRegistry instance, along with an interval of how often to run the checks. A registry requires a coroutine dispatcher to execute the checks on. Healthchecks can take advantage of coroutines to suspend if they need to do something IO based. Cohort will periodically run these healthchecks based on the passed schedule and record if they are healthy or unhealthy.

For example:

val checks = HealthCheckRegistry(Dispatchers.Default) {

  // detects if threads are mutually blocked on each others locks
  register(ThreadDeadlockHealthCheck(), 1.minutes)

// we should never have zero database connections
  register("reader connections", HikariConnectionsHealthCheck(ds, 1), 5.seconds)
}

With the registry created, we register it with Cohort by invoking the healthcheck method along with an endpoint url to expose it on.

For example:

install(Cohort) {
  healthcheck("/healthcheck", checks)
}

Whenever the endpoint is accessed, a 200 is returned if all health checks are currently reporting healthy, and a 500 otherwise.

Which healthchecks you use is entirely up to you, and you may want to use some healthchecks for startup probes, some for readiness checks and some for liveness checks. See the section on kubernetes for discussion on how to structure healthchecks in a kubernetes environment.

Available Healthchecks

This table lists the available health checks and their uses.

Kubernetes

A Kubernetes kubelet offers three kinds of probes to know the status of a container.

• liveness - Indicates whether the container is running. If the liveness probe fails, the kubelet kills the container (and restarts subject to the restart policy).
• readiness - Indicates whether the container is ready to respond to requests. If the readiness probe fails, the kubelet removes the pod from receiving traffic.
• startup - Indicates whether the application within the container has started. All other probes are disabled if a startup probe is provided, until it succeeds.

The kubelet uses liveness probes to know when to restart a container. Liveness probes help catch a situation where an application is running but is no longer useful. One such example is if a thread has stopped and the application does not have code to detect and restart the thread. Restarting a container in such a state can make the application available again despite the presence of bugs.

The kubelet uses readiness probes to know when a container should receive traffic. A pod is considered ready when all of its containers are ready. One use of this signal is to temporarily remove traffic from backends when they are unable to handle any more requests. For example, a service may have received more requests than it can handle, and so it's backlog of requests is growing. Taking that pod out of the load balancers while it catches up can avoid the service crashing or needing a restart. A pod with containers reporting that they are not ready does not receive traffic through Kubernetes Services.

Readiness probes are not a substitute for proper scaling (either HPA or manually) but they can avoid a situation where all pods are killed, and a service is completely unavailable.

The kubelet uses startup probes to know when a container application has fully started. If such a probe is configured, it disables liveness and readiness checks until it succeeds, making sure those probes don't interfere with the application startup. Startup probes are very useful if an application needs to perform slow initialization work and until that is complete, a liveness check would fail. This avoids situation where the failing liveness checks result in the kubelet killing the pod before it is ready.

Healthcheck Endpoint Output

Here is an example of output from a health check with a series of configured health checks.

[
  {
    "name": "com.sksamuel.cohort.memory.FreememHealthCheck",
    "healthy": true,
    "lastCheck": "2022-03-15T03:01:09.445932Z",
    "message": "Freemem is above threshold [433441040 >= 67108864]",
    "cause": null,
    "consecutiveSuccesses": 75,
    "consecutiveFailures": 0
  },
  {
    "name": "com.sksamuel.cohort.system.OpenFileDescriptorsHealthCheck",
    "healthy": true,
    "lastCheck": "2022-03-15T03:01:09.429469Z",
    "message": "Open file descriptor count within threshold [209 <= 16000]",
    "cause": null,
    "consecutiveSuccesses": 25,
    "consecutiveFailures": 0
  },
  {
    "name": "com.sksamuel.cohort.memory.GarbageCollectionTimeCheck",
    "healthy": true,
    "lastCheck": "2022-03-15T03:00:54.422194Z",
    "message": "GC Collection time was 0% [Max is 25]",
    "cause": null,
    "consecutiveSuccesses": 6,
    "consecutiveFailures": 0
  },
  {
    "name": "writer connections",
    "healthy": true,
    "lastCheck": "2022-03-15T03:01:09.445868Z",
    "message": "Database connections is equal or above threshold [8 >= 8]",
    "cause": null,
    "consecutiveSuccesses": 75,
    "consecutiveFailures": 0
  },
  {
    "name": "reader connections",
    "healthy": true,
    "lastCheck": "2022-03-15T03:01:09.445841Z",
    "message": "Database connections is equal or above threshold [8 >= 8]",
    "cause": null,
    "consecutiveSuccesses": 75,
    "consecutiveFailures": 0
  },
  {
    "name": "com.sksamuel.cohort.system.SystemCpuHealthCheck",
    "healthy": true,
    "lastCheck": "2022-03-15T03:01:09.463421Z",
    "message": "System CPU is below threshold [0.12667261373773417 < 0.9]",
    "cause": null,
    "consecutiveSuccesses": 75,
    "consecutiveFailures": 0
  },
  {
    "name": "com.sksamuel.cohort.threads.ThreadDeadlockHealthCheck",
    "healthy": true,
    "lastCheck": "2022-03-15T03:00:54.419733Z",
    "message": "There are 0 deadlocked threads",
    "cause": null,
    "consecutiveSuccesses": 6,
    "consecutiveFailures": 0
  }
]

Logging

Cohort allows you to view the current logging configuration and update log levels at runtime.

To enable this, pass an instance of the LogManager interface for the logging framework you are using to the logManagerparameter in the Cohort plugin configuration.

Once enabled, the endpoint GET /cohort/logging can be used to show current log information and PUT /cohort/logging/{name}/{level} can be used to modify a log level at runtime.

Cohort currently supports two LogManager implementations:

• LogbackManager - add module com.sksamuel.cohort:cohort-logback:<version>
• Log4j2Manager - add module com.sksamuel.cohort:cohort-log4j2:<version>

For example, for projects that use logback, you can configure like this:

install(Cohort) {
  logManager = LogbackManager
}

Here is the example output of which shows the logging configuration:

{
  "levels": [
    "DEBUG",
    "TRACE",
    "INFO",
    "ERROR",
    "OFF",
    "WARN"
  ],
  "loggers": [
    {
      "name": "ROOT",
      "level": "INFO"
    },
    {
      "name": "com",
      "level": "INFO"
    },
    {
      "name": "com.sksamuel",
      "level": "INFO"
    },
    {
      "name": "ktor",
      "level": "INFO"
    },
    {
      "name": "ktor.application",
      "level": "INFO"
    },
    {
      "name": "org",
      "level": "INFO"
    },
    {
      "name": "org.apache",
      "level": "INFO"
    },
    {
      "name": "org.apache.kafka",
      "level": "WARN"
    }
  ]
}

Jvm Info

Displays information about the JVM state, including VM options, JVM version, and vendor name.

To enable, set jvmInfo to true inside the Cohort ktor configuration block:

install(Cohort) {
  jvmInfo = true
}

{
  "name": "106637@sam-H310M-A-2-0",
  "pid": 106637,
  "vmOptions": [
    "-Dvisualvm.id=32227655111670",
    "-javaagent:/home/sam/development/idea-IU-213.5744.125/lib/idea_rt.jar=36667:/home/sam/development/idea-IU-213.5744.125/bin",
    "-Dfile.encoding=UTF-8"
  ],
  "classPath": "/home/sam/development/workspace/......",
  "specName": "Java Virtual Machine Specification",
  "specVendor": "Oracle Corporation",
  "specVersion": "11",
  "vmName": "OpenJDK 64-Bit Server VM",
  "vmVendor": "AdoptOpenJDK",
  "vmVersion": "11.0.10+9",
  "startTime": 1647315704746,
  "uptime": 405278
}

Operating System

Displays the running os and version.

To enable, set operatingSystem to true inside the Cohort ktor configuration block:

install(Cohort) {
  operatingSystem = true
}

{
  "arch": "amd64",
  "name": "Linux",
  "version": "5.13.0-35-generic"
}

Datasources

By passing one or more database pools to Cohort, you can see at runtime the current state of the pool(s). Once enabled, a GET request to /cohort/datasources will return information such as idle connection count, max pool size, connection timeouts and so on.

Cohort supports two connection pool libraries:

• Apache Commons DBCP - add module com.sksamuel.cohort:cohort-dbcp
• HikariCP - add module com.sksamuel.cohort:cohort-hikari

To activate this feature, wrap your DataSource in an appropriate DataSourceManager instance and pass through to the Cohort plugin.

For example, if we had two connection pools, a writer pool using Hikari, and a reader pool using Apache DBCP, then we could configure like this:

install(Cohort) {
  dataSources = listOf(
    ApacheDBCPDataSourceManager(reader),
    HikariDataSourceManager(writer),
  )
}

Here is an example output for the above datasources:

[
  {
    "name": "writer",
    "activeConnections": 0,
    "idleConnections": 8,
    "totalConnections": 8,
    "threadsAwaitingConnection": 0,
    "connectionTimeout": 30000,
    "idleTimeout": 600000,
    "maxLifetime": 1800000,
    "leakDetectionThreshold": 0,
    "maximumPoolSize": 16,
    "validationTimeout": 5000
  },
  {
    "name": "reader",
    "activeConnections": 0,
    "idleConnections": 8,
    "totalConnections": 8,
    "threadsAwaitingConnection": 0,
    "connectionTimeout": 30000,
    "idleTimeout": 600000,
    "maxLifetime": 1800000,
    "leakDetectionThreshold": 0,
    "maximumPoolSize": 16,
    "validationTimeout": 5000
  }
]

System Properties

Send a GET request to /cohort/sysprops to return the current system properties.

To enable, set sysprops to true inside the Cohort plugin configuration block:

install(Cohort) {
  sysprops = true
}

Here is an example of the output:

{
  "sun.jnu.encoding": "UTF-8",
  "java.vm.vendor": "AdoptOpenJDK",
  "java.vendor.url": "https://adoptopenjdk.net/",
  "user.timezone": "America/Chicago",
  "os.name": "Linux",
  "java.vm.specification.version": "11",
  "user.country": "US",
  "sun.boot.library.path": "/home/sam/.sdkman/candidates/java/11.0.10.hs-adpt/lib",
  "sun.java.command": "com.myapp.MainKt",
  "user.home": "/home/sam",
  "java.version.date": "2021-01-19",
  "java.home": "/home/sam/.sdkman/candidates/java/11.0.10.hs-adpt",
  "file.separator": "/",
  "java.vm.compressedOopsMode": "Zero based",
  "line.separator": "\n",
  "java.specification.name": "Java Platform API Specification",
  "java.vm.specification.vendor": "Oracle Corporation",
  "sun.management.compiler": "HotSpot 64-Bit Tiered Compilers",
  "java.runtime.version": "11.0.10+9",
  "user.name": "sam",
  "path.separator": ":",
  "file.encoding": "UTF-8",
  "java.vm.name": "OpenJDK 64-Bit Server VM",
  "user.dir": "/home/sam/development/workspace/myapp",
  "os.arch": "amd64",
  "java.vm.specification.name": "Java Virtual Machine Specification",
  "java.awt.printerjob": "sun.print.PSPrinterJob",
  "java.class.version": "55.0"
}

Heap Dump

Send a GET request to /cohort/heapdump to retrieve a heap dump for all live objects.

The file returned is in the format used by hprof.

To enable, set heapdump to true inside the Cohort plugin configuration block:

install(Cohort) {
  heapdump = true
}

Thread Dump

Send a GET request to /cohort/threaddump to retrieve a thread dump for all current threads.

To enable, set threaddump to true inside the Cohort plugin configuration block:

install(Cohort) {
  threaddump = true
}

Example output:

"main" prio=5 Id=1 WAITING on io.netty.channel.AbstractChannel$CloseFuture@291c536c
	at [email protected]/java.lang.Object.wait(Native Method)
	-  waiting on io.netty.channel.AbstractChannel$CloseFuture@291c536c
	at [email protected]/java.lang.Object.wait(Object.java:328)
	at app//io.netty.util.concurrent.DefaultPromise.await(DefaultPromise.java:253)
	at app//io.netty.channel.DefaultChannelPromise.await(DefaultChannelPromise.java:131)
	at app//io.netty.channel.DefaultChannelPromise.await(DefaultChannelPromise.java:30)
	at app//io.netty.util.concurrent.DefaultPromise.sync(DefaultPromise.java:404)
	at app//io.netty.channel.DefaultChannelPromise.sync(DefaultChannelPromise.java:119)
	at app//io.netty.channel.DefaultChannelPromise.sync(DefaultChannelPromise.java:30)
	...

"Reference Handler" daemon prio=10 Id=2 RUNNABLE
	at [email protected]/java.lang.ref.Reference.waitForReferencePendingList(Native Method)
	at [email protected]/java.lang.ref.Reference.processPendingReferences(Reference.java:241)
	at [email protected]/java.lang.ref.Reference$ReferenceHandler.run(Reference.java:213)

"Finalizer" daemon prio=8 Id=3 WAITING on java.lang.ref.ReferenceQueue$Lock@1392e5c7
	at [email protected]/java.lang.Object.wait(Native Method)
	-  waiting on java.lang.ref.ReferenceQueue$Lock@1392e5c7
	at [email protected]/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:155)
	at [email protected]/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:176)
	at [email protected]/java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:170)

"Signal Dispatcher" daemon prio=9 Id=4 RUNNABLE

"Common-Cleaner" daemon prio=8 Id=19 TIMED_WAITING on java.lang.ref.ReferenceQueue$Lock@78e1959d
	at [email protected]/java.lang.Object.wait(Native Method)
	-  waiting on java.lang.ref.ReferenceQueue$Lock@78e1959d
	at [email protected]/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:155)
	at [email protected]/jdk.internal.ref.CleanerImpl.run(CleanerImpl.java:148)
	at [email protected]/java.lang.Thread.run(Thread.java:834)
	at [email protected]/jdk.internal.misc.InnocuousThread.run(InnocuousThread.java:134)